Oscar Pistorius must undergo psychiatric tests, the judge in his murder trial has ruled.

The move is likely to lead to significant delays in the case, but Judge Thokozile Masipa said it was about justice, not convenience.

She said Pistorius could be evaluated as an out-patient, indicating he would not have to be admitted to a psychiatric hospital.

Further details about the referral are likely to come out when the order is officially made on Tuesday.

The prosecution made the application for an assessment after defence evidence from a forensic psychiatrist, who said the defendant had generalised anxiety disorder.

But the defence argued there was no need for psychiatric observation and the application had "no merit".

The prosecution has already claimed the decision to call Dr Merryl Vorster so late in the trial - after Pistorius himself had given evidence - may be a "fallback option" for the defence in the event of a guilty verdict.

Sending the 27-year-old for mental health tests at this stage could eliminate the possibility of an appeal by the defence on the grounds that Pistorius' mental health had not been fully and adequately assessed.

If Pistorius were found to be suffering from a mental illness, he could be held not criminally responsible for his girlfriend Reeva Steenkamp's death and found not guilty by reason of "mental illness or intellectual disability".

Judge Masipa said the court was "ill-equipped" to assess the diagnosis by Dr Vorster, and so ruled Pistorius should be sent for evaluation.

She said: "The accused may not have raised the issue that he was not criminally responsible at the time of the incident in so many words, but evidence led on his behalf clearly raised the issue and cannot be ignored."

In her evidence, Dr Vorster said Pistorius had GAD and was a "distrusting and guarded" person who is "hyper-vigilant" about security.

Video:Pistorius Trial Special Programme

But she also said he was able to able "to function at a high-level", and did socialise.

Pistorius is accused of killing Ms Steenkamp in a premeditated attack at his home in Pretoria, South Africa, on Valentine's Day last year.

He denies the charge and claims he shot his partner after mistaking her for an intruder.